This question has a slight undertone to it that harkens back to the term "Real Man". Legally according to the law any male that is age 18 or older is considered a man. In other words if he got arrested he would be tried as an adult.
However I suspect you are asking how can you tell if a guy is "mature" and "responsible". If he does what he says he will do, takes care of his obligations, and is willing to admit he is wrong if he has made a mistake those are the key traits to being a (good) man. Needless to say a guy who is an abuser, irresponsible, or immature is still a man with those traits. To call him a boy is an insult to actual boys. There is such a thing as being a "bad man".
I personally have never understood the need for some people to go around labeling other people. More often than not those are the people that are looking to "manipulate" someone. They try to brow beat them into doing what they want them to do. "A real man would ....etc" Truth be told a "real man" would not let someone else define who he is.
All that matters is you spend your time with someone that has the traits (YOU) are looking for. If you determine a guy does not have them then you move on. One woman's trash is another woman's treasure and vice versa. Ultimately we're all looking for someone that (naturally agrees) with us on the major things in life.